The solar system appears to have a 'new' ninth planet (the 'old'
nine planet was Pluto till it
was 'declassified'
as such...)

Today, two scientists announced evidence
that a body nearly the size of Neptune - but as yet unseen - orbits
the sun every 15,000 years. During the solar system's infancy 4.5
billion years ago, they say, the giant planet was knocked out of the
planet-forming region near the sun.

Slowed down by gas, the planet settled
into a distant elliptical orbit, where it still lurks today.

The claim is the strongest yet in the centuries-long search for a "Planet
X" beyond Neptune. The quest has been plagued by
far-fetched claims and even outright quackery.

But the new evidence comes from a pair
of respected planetary scientists, Konstantin Batygin and
Mike Brown of the California Institute of Technology (Caltech)
in Pasadena, who prepared for the inevitable skepticism with
detailed analyses of the orbits of other distant objects and months
of computer simulations.

"If you say, 'We have evidence for
Planet X,' almost any astronomer will say, 'This again? These
guys are clearly crazy.' I would, too," Brown says.

"Why is this different? This is
different because this time we're right."

Outside scientists say their calculations stack up and express a
mixture of caution and excitement about the result.

"I could not imagine a bigger deal
if - and of course that's a boldface 'if' - if it turns out to
be right," says Gregory Laughlin, a planetary scientist at the
University of California (UC), Santa Cruz.

"What's thrilling about it is [the
planet] is detectable."

Batygin and Brown inferred its presence
from the peculiar clustering of six previously known objects that
orbit beyond Neptune.

They say there's only a 0.007% chance,
or about one in 15,000, that the clustering could be a coincidence.
Instead, they say, a planet with the mass of 10 Earths has
shepherded the six objects into their strange elliptical orbits,
tilted out of the plane of the solar system.

The orbit of the inferred planet is similarly tilted, as well as
stretched to distances that will explode previous conceptions of the
solar system.

Its closest approach to the sun is seven
times farther than Neptune, or 200 astronomical units - AUs - (an AU
is the distance between Earth and the sun, about 150 million
kilometers.)

And Planet X could roam as far as 600 to
1200 AU, well beyond the Kuiper belt, the region of small icy worlds
that begins at Neptune's edge about 30 AU. If
Planet X is out there, Brown and
Batygin say, astronomers ought to find more objects in telltale
orbits, shaped by the pull of the hidden giant.

But Brown knows that no one will really
believe in the discovery until Planet X itself appears within a
telescope viewfinder.

"Until there's a direct detection,
it's a hypothesis - even a potentially very good
hypothesis," he says.

The team has time on the one large
telescope in Hawaii that is suited for the search, and they hope
other astronomers will join in the hunt.

Alessandro Morbidelli, a
planetary dynamicist at the Nice Observatory in France, performed
the peer review for the paper. In a statement, he says Batygin and
Brown made a "very solid argument" and that he is "quite convinced
by the existence of a distant planet."

Championing a 'new' ninth planet is an ironic role for Brown;
he is better known as a planet slayer.

His 2005 discovery of
Eris, a remote icy world nearly the
same size as Pluto, revealed that what was seen as the outermost
planet was just one of many worlds in
the Kuiper belt.

Now, he has joined the centuries-old search for new planets. His
method - inferring the existence of Planet X from its ghostly
gravitational effects - has a respectable track record.

In 1846, for example, the French
mathematician Urbain Le Verrier predicted the existence of a
giant planet from irregularities in the orbit of Uranus. Astronomers
at the Berlin Observatory found the new planet, Neptune, where it
was supposed to be, sparking a media sensation.

Remaining hiccups in Uranus's orbit led scientists to think that
there might yet be one more planet, and in 1906 Percival Lowell,
a wealthy tycoon, began the search for what he called "Planet X" at
his new observatory in Flagstaff, Arizona.

In 1930, Pluto turned up - but it was
far too small to tug meaningfully on Uranus. More than half a
century later, new calculations based on measurements by the Voyager
spacecraft revealed that the orbits of Uranus and Neptune were just
fine on their own: No Planet X was needed.

In the 1980s, for example, researchers
proposed that an unseen brown dwarf star could cause periodic
extinctions on Earth by triggering fusillades of comets. In the
1990s, scientists invoked a Jupiter-sized planet at the solar
system's edge to explain the origin of certain oddball comets.

Just last month, researchers claimed to
have detected the faint microwave glow of an outsized rocky planet
some 300 AU away, using an array of telescope dishes in Chile called
the Atacama Large Millimeter Array (ALMA).
(Brown was one of many skeptics, noting that ALMA's narrow field of
view made the chances of finding such an object vanishingly slim.)

Brown got his first inkling of his current quarry in 2003, when he
led a team that found Sedna, an object a bit
smaller than both Eris and Pluto.

Sedna's odd, far-flung orbit made it the
most distant known object in the solar system at the time. Its
perihelion, or closest point to the sun, lay at 76 AU, beyond the
Kuiper belt and far outside the influence of Neptune's gravity.

The implication was clear:

Something massive, well beyond
Neptune, must have pulled Sedna into its distant orbit.

(DATA) JPL; BATYGIN AND BROWN/CALTECH;

(DIAGRAM) A. CUADRA/SCIENCE

That something didn't have to be a planet.

Sedna's gravitational nudge could have
come from a passing star, or from one of the many other stellar
nurseries that surrounded the nascent sun at the time of the solar
system's formation.

Since then, a handful of other icy objects have turned up in similar
orbits.

By combining Sedna with five other
weirdos, Brown says he has ruled out stars as the unseen influence:

Only a planet could explain such
strange orbits.

Of his three major discoveries - Eris,
Sedna, and now, potentially, Planet X - Brown says the last is the
most sensational.

"Killing Pluto was fun. Finding
Sedna was scientifically interesting," he says. "But this one,
this is head and shoulders above everything else."

Brown and Batygin were nearly beaten to
the punch.

For years, Sedna was a lone clue to a
perturbation from beyond Neptune. Then, in 2014, Scott Sheppard
and Chad Trujillo (a former graduate student of Brown's)
published a paper describing the discovery of
VP113, another object that never
comes close to the sun.

Sheppard, of the Carnegie Institution
for Science in Washington, D.C., and Trujillo, of the Gemini
Observatory in Hawaii, were well aware of the implications.

They began to examine the orbits of the
two objects along with 10 other oddballs. They noticed that, at
perihelion, all came very near the plane of solar system in which
Earth orbits, called
the ecliptic.

In a paper, Sheppard and Trujillo
pointed out the peculiar clumping and raised the possibility that a
distant large planet had herded the objects near the ecliptic. But
they didn't press the result any further.

Later that year, at Caltech, Batygin and Brown began discussing the
results.

Plotting the orbits of the distant
objects, Batygin says, they realized that the pattern that Sheppard
and Trujillo had noticed "was only half of the story." Not only were
the objects near the ecliptic at perihelia, but their perihelia were
physically clustered in space (see diagram, above).

For the next year, the duo secretly discussed the pattern and what
it meant. It was an easy relationship, and their skills complemented
each other. Batygin, a 29-year-old whiz kid computer modeler, went
to college at UC Santa Cruz for the beach and the chance to play in
a rock band.

But he made his mark there by modeling
the fate of the solar system over billions of years, showing that,
in rare cases, it was unstable: Mercury may plunge into the sun or
collide with Venus.

"It was an amazing accomplishment
for an undergraduate," says Laughlin, who worked with him at the
time.

Brown, 50, is the observational
astronomer, with a flair for dramatic discoveries and the confidence
to match.

He wears shorts and sandals to work,
puts his feet up on his desk, and has a breeziness that masks
intensity and ambition. He has a program all set to sift for Planet
X in data from a major telescope the moment they become publicly
available later this year.

Their offices are a few doors down from each other.

"My couch is nicer, so we tend to
talk more in my office," Batygin says. "We tend to look more at
data in Mike's."

They even became exercise buddies, and
discussed their ideas while waiting to get in the water at a Los
Angeles, California, triathlon in the spring of 2015.

First, they winnowed the dozen objects studied by Sheppard and
Trujillo to the six most distant - discovered by six different
surveys on six different telescopes. That made it less likely that
the clumping might be due to an observation bias such as pointing a
telescope at a particular part of the sky.

Batygin began seeding his solar system models with Planet X's of
various sizes and orbits, to see which version best explained the
objects' paths. Some of the computer runs took months.

A favored size for Planet X emerged -
between five and 15 Earth masses - as well as a preferred orbit:

anti-aligned in space from the six
small objects, so that its perihelion is in the same direction
as the six objects' aphelion, or farthest point from the sun.

The orbits of the six cross that of
Planet X, but not when the big bully is nearby and could disrupt
them.

The final epiphany came 2 months ago,
when Batygin's simulations showed that Planet X should also sculpt
the orbits of objects that swoop into the solar system from above
and below, nearly orthogonal to the ecliptic.

"It sparked this memory," Brown
says. "I had seen these objects before."

It turns out that, since 2002, five of
these highly inclined Kuiper belt objects have been discovered, and
their origins are largely unexplained.

"Not only are they there, but they
are in exactly the places we predicted," Brown says. "That is
when I realized that this is not just an interesting and good
idea - this is actually real."

Sheppard, who with Trujillo had also
suspected an unseen planet, says Batygin and Brown,

"took our result to the next level…
They got deep into the dynamics, something that Chad and I
aren't really good with. That's why I think this is exciting."

Others, like planetary scientist Dave
Jewitt, who discovered the Kuiper belt, are more cautious.

The 0.007% chance that the clustering of
the six objects is coincidental gives the planet claim a statistical
significance of 3.8 sigma - beyond the 3-sigma threshold typically
required to be taken seriously, but short of the 5 sigma that is
sometimes used in fields like particle physics.

That worries Jewitt, who has seen plenty
of 3-sigma results disappear before.

By reducing the dozen objects examined
by Sheppard and Trujillo to six for their analysis, Batygin and
Brown weakened their claim, he says.

"I worry that the finding of a
single new object that is not in the group would destroy the
whole edifice," says Jewitt, who is at UC Los Angeles. "It's a
game of sticks with only six sticks."

At first blush, another potential problem comes from NASA's
Widefield Infrared Survey Explorer (WISE),
a satellite that completed an all-sky survey looking for the heat of
brown dwarfs - or giant planets.

It ruled out the existence of a
Saturn-or-larger planet as far out as 10,000 AU, according to a 2013
study by Kevin Luhman, an astronomer at Pennsylvania State
University, University Park.

But Luhman notes that if Planet X is
Neptune-sized or smaller, as Batygin and Brown say, WISE would have
missed it. He says there is a slim chance of detection in another
WISE data set at longer wavelengths - sensitive to cooler radiation
- which was collected for 20% of the sky.

Luhman is now analyzing those data.

Even if Batygin and Brown can convince other astronomers that Planet
X exists, they face another challenge: explaining how it ended up so
far from the sun. At such distances, the protoplanetary disk of dust
and gas was likely to have been too thin to fuel planet growth.

And even if Planet X did get a foothold
as a planetesimal, it would have moved too slowly in its vast, lazy
orbit to hoover up enough material to become a giant.

Instead, Batygin and Brown propose that Planet X formed much closer
to the sun, alongside Jupiter, Saturn, Uranus, and Neptune. Computer
models have shown that the early solar system was a tumultuous
billiards table, with dozens or even hundreds of planetary building
blocks the size of Earth bouncing around.

Another embryonic giant planet could
easily have formed there, only to be booted outward by a
gravitational kick from another gas giant. It's harder to explain
why Planet X didn't either loop back around to where it started or
leave the solar system entirely.

But Batygin says that residual gas in
the protoplanetary disk might have exerted enough drag to slow the
planet just enough for it to settle into a distant orbit and remain
in the solar system.

That could have happened if the ejection
took place when the solar system was between 3 million and 10
million years old, he says, before all the gas in the disk was lost
into space.

Hal Levison, a planetary dynamicist at the Southwest Research
Institute in Boulder, Colorado, agrees that something has to be
creating the orbital alignment Batygin and Brown have detected. But
he says the origin story they have developed for Planet X and their
special pleading for a gas-slowed ejection add up to "a
low-probability event."

Other researchers are more positive.

The proposed scenario is plausible,
Laughlin says.

"Usually things like this are wrong,
but I'm really excited about this one," he says. "It's better
than a coin flip."

All this means that Planet X will remain
in limbo until it is actually found.

Astronomers have some good ideas about where to look, but spotting
the new planet won't be easy. Because objects in highly elliptical
orbits move fastest when they are close to the sun, Planet X spends
very little time at 200 AU. And if it were there right now, Brown
says, it would be so bright that astronomers probably would have
already spotted it.

Instead, Planet X is likely to spend most of its time near aphelion,
slowly trotting along at distances between 600 and 1200 AU.

Most telescopes capable of seeing a dim
object at such distances, such as the Hubble Space Telescope or the
10-meter Keck telescopes in Hawaii, have extremely tiny fields of
view. It would be like looking for a needle in a haystack by peering
through a drinking straw.

It has enough light-gathering area to
detect such a faint object, coupled with a huge field of view - 75
times larger than that of a Keck telescope. That allows astronomers
to scan large swaths of the sky each night.

Batygin and Brown are using Subaru to
look for Planet X - and they are coordinating their efforts with
their erstwhile competitors, Sheppard and Trujillo, who have also
joined the hunt with Subaru.

Brown says it will take about 5 years
for the two teams to search most of the area where Planet X could be
lurking.

The 8-meter Subaru Telescope atop Mauna Kea in Hawaii

has a large field of
view - enabling it

to search efficiently
for Planet X.
Subaru Telescope, NAOJ

If the search pans out, what should the new member of the sun's
family be called?

Brown says it's too early to worry about
that and scrupulously avoids offering up suggestions. For now, he
and Batygin are calling it 'Planet Nine' (and, for the past
year, informally, Planet Phattie - 1990s slang for "cool").

Brown notes that neither Uranus nor
Neptune - the two planets discovered in modern times - ended up
being named by their discoverers, and he thinks that that's probably
a good thing.

It's bigger than any one person, he
says:

"It's kind of like finding a new
continent on Earth."

He is sure, however, that Planet X -
unlike Pluto - deserves to be called aplanet.